CLUTCH SIZE, NEST PREDATION, AND DISTRIBUTION OF AVIAN UNEQUAL COMPETITORS IN A PATCHY ENVIRONMENT

I studied reproductive success, nest predation, and most productive clutch size of seven passerine bird species (Barn Swallow Hirundo rustica, Great Tit Parus major, European Blackbird Turdus merula, Yellowhammer Emberiza citrinella, House Sparrow Passer domesticus, Chaffinch Fringilla coelebs, Black-billed Magpie Pica pica) breeding in a patchy environment of tree lots in a matrix of farmland. Habitat patches of 0.07-3.61 ha consisted primarily of elm Ulmus glabra and various other deciduous trees, a thick undergrowth of bushes, primarily elder Sambucus nigra, and herbs. Clutch size of all species was independent of habitat patch size even after controlling for time of breeding and age of breeding birds, but brood size at fledging increased with increasing patch size for the Great Tit, European Blackbird, Yellowhammer, Chaffinch, and Black-billed Magpie, but not for the Barn Swallow and House Sparrow. The frequency of nest predation decreased with increasing size of habitat patches in the open-nesting European Blackbird, Yellow-hammer, Chaffinch, and Black-billed Magpie, but not in the hole- or the semi-hole nesters. Brood size of Great Tit, European Blackbird, Yellowhammer, Chaffinch, and Black-billed Magpie increased with increasing patch size even after excluding nests suffering from nest predation. The most productive clutch size measured as the second-order polynomial regression of brood size at fledging on clutch size differed in relation to habitat patch size for the Great Tit, European Blackbird, Yellowhammer, Chaffinch, and Black-billed Magpie, but not for the Barn Swallow and House Sparrow, suggesting that there is no unique optimal clutch size in the first five species. A decreasing fraction of yearling breeding birds was found with increasing patch size in the Great Tit, European Blackbird, Yellowhammer, and Chaffinch, but not in the Barn Swallow and House Sparrow. Habitat patches differed in quality with respect to absolute food abundance and nest predation risks, and territorial species relying nearly exclusively on food resources found within patches or suffering from intense nest predation followed an ideal despotic distribution, while colonial species relying on food resources outside patches followed an ideal free distribution for unequal competitors.